Lectins are highly specific carbohydrate-binding proteins. Lectins have been isolated from a variety of natural sources including seeds, roots, bark, fungi, bacteria, seaweed, sponges, mollusks, fish eggs, body fluids of invertebrates and lower vertebrates, and mammalian cell membranes (e.g., see The Lectins: Properties, Functions, and Applications in Biology and Medicine, Edited by Liener et al., Academic Press, 1986). A number of lectins have also been produced recombinantly (e.g., see Streicher and Sharon, Methods Enzymol. 363:47-77, 2003). As noted, lectins bind carbohydrates with a high degree of specificity. For example, some lectins will bind only to carbohydrates with mannose or glucose residues, while others only recognize carbohydrates that include galactose residues. Some lectins require that the particular residue be in a terminal position, while others bind to residues within a carbohydrate chain. Some lectins require specific anomeric structures and yet others recognize specific sugar sequences. The structures and properties of lectins have been extensively described in the literature. For recent reviews see Lectins, Edited by Sharon and Lis, Kluwer Academic Publishers, 2003; Handbook of Animal Lectins: Properties and Biomedical Applications, Edited by Kilpatrick, Wiley, 2000; and Handbook of Plant Lectins: Properties and Biomedical Applications, Edited by Van Damme et al., Wiley, 1998. Each of these references and any other publication that is cited herein is hereby incorporated by reference.
Most lectins studied to date are multimeric, consisting of non-covalently associated subunits. While some multimeric lectins include two or more identical subunits (e.g., Concanavalin A), others are composed of different subunits (e.g., Phaseolus vulgaris). Each subunit can bind an independent carbohydrate structure. The multivalent binding properties of multimeric lectins allow them to agglutinate cells by binding to carbohydrate structures on multiple cell surfaces. For this reason, cellular agglutination assays are commonly used to identify novel lectins. Although most lectins can agglutinate some cell type, cellular agglutination is not a prerequisite. In particular some lectins are monovalent while others do not recognize cell-surface carbohydrate structures. Most multivalent lectins are also capable of forming precipitates with glycoconjugates (e.g., complex carbohydrates, glycoproteins, glycolipids, etc.).
Lectins bind carbohydrate structures non-covalently. The binding in a lectin-carbohydrate complex can therefore be reversed or inhibited by competing monosaccharides such as glucose (and even certain di-, tri- or polysaccharides). A variety of methods and devices have been described in the art that take advantage of the reversible carbohydrate binding properties of lectins. For example, sensors that monitor sugar concentrations using lectins have been described (e.g., see U.S. Pat. Nos. 5,814,449; 6,454,710 and 6,671,527). Devices that rely on lectins to deliver drugs such as insulin in a controlled and glucose responsive manner have also been described (e.g., see U.S. Pat. Nos. 5,830,506; 5,902,607 and 6,410,053).
Unfortunately, many of the most readily available lectins pose great risks in vivo because they have the potential to stimulate lymphocyte proliferation (e.g., without limitation, Artocarpus integrifolia agglutinin (Jacalin), Bauhinia purpurea agglutinin (BPA), Concanavalin A (Con A), succinyl-Concanavalin A (s-Con A), Erythrina corallodendron agglutinin (ECorA), Euonymus europaeus agglutinin (EEA), Glycine max agglutinin (SBA), Lens culinaris agglutinin (LcH), Maackia amurensis agglutinin (MAA), Phaseolus vulgaris agglutinin (PHA), Pokeweed mitogen (PWM), Wheat germ agglutinin (WGA), and Vicia faba agglutinin (VFA) all of which are available from Sigma-Aldrich of St. Louis, Mo.). By binding to carbohydrate receptors on the surfaces of certain types of lymphocytes, these and other so-called “mitogenic” lectins induce the mitosis of lymphocytes and thereby cause them to proliferate. Most mitogenic lectins including Con A and PHA are selective T-cell mitogens. A few lectins such as PWM are less selective and stimulate both T-cells and B-cells. Local or systemic in vivo exposure to mitogenic lectins results in inflammation, cytotoxicity, macrophage digestion, and allergic reactions including anaphylaxis. In addition, plant lectins are known to be highly immunogenic, giving rise to the production of high titers of anti-lectin specific antibodies.
It will be appreciated that mitogenic lectins cannot therefore be used for in vivo methods and devices unless great care is taken to prevent their release. For example, in U.S. Pat. No. 5,830,506, the inventor highlights the toxic risks that are involved in using Con A and emphasizes the importance and difficulty of containing Con A within a drug delivery device that also requires glucose and insulin molecules to diffuse freely in and out of the device.
The risks and difficulties that are involved with these and other in vivo uses of lectins could be significantly diminished if the mitogenicity of lectin compositions could be reduced or even eliminated. There is therefore a need in the art for methods of reducing the T-cell mitogenicity of lectin compositions. In particular, there is a need for methods that reduce the T-cell mitogenicity of lectins under physiological conditions. There is also a need for methods that do not adversely affect the carbohydrate binding properties of lectins.